Carbon purification process



March 17, 1970 J. L. BENAK CARBON PURIFICATION PROCESS Filed Feb. 281966 SAMPLES REMOVED HERE g s &r r 001mm wen: TOP\ & EPOX RE5IN PURECARBON F020 GLASS FIBER MAT SEPARATOR RTV88 SIUCONE RUBBE'L. F1 AJ\\\\\\\\\ L -ITE BOTTOM,

PM" GRAPHITE ROD CURRENT COLLECTOR (NEW DRACKISHHON-CONTAINING couuscrwTO FC 20 WITH WATER ENTERS HERE SOLU (MILLILITERS) DESALINATION 0F PPMNACL ON 0100 700 mg 4' VOLU E PU PED NAT. CARBON c-9 CEMENT CONCENTRATEDSALT WATER LEAvEs HERE '2 59 CALCIUM CHLORIDE REMOVAL :3 4200 5% p- 1000555 000 i Q; 600 400 P E" E o CHARGE 0 2o so 00100120140 3 VOLUME PUMPEDg- (MILLILITERS) 2 CALCIUM SULPHATE 00 REMOVAL Ii 3 500 5% 400 0 II umnu-rm I III" INVENTOR- e/AMES 5NAK nv I' k/dq ATTO NEVJ United StatesPatent 3,501,272 CARBON PURIFICATION PROCESS James L. Benak, Cleveland,Ohio, assignor to The Standard Oil Company, Cleveland, Ohio, acorporation of Ohio Filed Feb. 28, 1966, Ser. No. 530,681 Int. Cl. C01b31/02 US. Cl. 23-2099 4 Claims ABSTRACT OF THE DISCLOSURE The inventiondescribed herein is a process for the purification of carbon. Thatprocess provides for the treatment of carbon with hydrofluoric acid, ora mixture of hydrofluoric acid and nitric acid, followed in each case bytreatment with hot aqueous hydrochloric acid. The carbon thus purifiedis notably effective for use as electrode material in a demineralizationcell, i.e., one used for the demineralization of water.

This invention relates to a novel process for purifying and refiningcarbon to produce novel carbons. Still further the invention relates tothe use of the novel carbons in the demineralization of water.

THE PROBLEM Carbon is a plentiful substance. It occurs mainly in naturalforms such as wood and the like. In addition to carbon, these precursorscontain several non-carbon materials, making up the total ash oncombustion. The amount of ash can vary substantially. Thus, tree wood-smay contain 3% or more of ash. On the other hand, coconut hulls maycontain as low as .55% ash. In addition to various refractory oxides,the ash commonly contains silica (SiO and graphitoidal silicon (Si incertain crystalline state). Sorne char carbons contain bothcontaminants, while others contain one or the other of the contaminants.For many applications, such as electrodes and the like, the presence ofthe ash components is deleterious.

There have been many attempts in the prior art to remove the silica and/or graphitoidal silicon so that the carbon might have application formore sophisticated uses than would otherwise be possible.

THE DEMINERALIZATION ASPECT A major problem that has been encountered inthe development of a double-layer water demineralization apparatus hasbeen the clogging of the electrodes, which reduces their efficiencybelow practical levels in a relatively short period of time. In thistype of demineralization cell, porous carbon electrodes are insertedlll'tO a body of water and a carefully selected electrical potentialisapplied to attract the mineral ions contained in the water to thesurfaces of the electrodes. The ions are held in opposed relation to anelectron charge in the electrode in a double-layer condition, analogousto the storage of electrical energy in a capacitor.

After a given interval, the current is reversed and the retained ionsare theoretically discharged in a wash stream of water to clean theelectrodes so that a repeat demineralization cycle can then be effected.

The problem was however that due to some unknown component in thestructure of the carbon, such apparently as silica and ash, thecollected mineral content from the'water could not be releasedeffectively. The result was that the electrodes clogged badly and soonbecame inoperable.

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CONTRIBUTION TO THE ART Accordingly, a substantial contribution to theart would be provided by a unique electrode carbon applicable for use inthe electrostatic demineralization of water.

Further, a substantial contribution to the art would be provided by anovel process for treating carbons to Eliminate silica and/orgraphitoidal silicon and ash thererom.

Still further, a substantial contribution to the art would be providedby a novel water demineralization process utilizing electrodes preparedin accordance with the present invention.

In view of the foregoing, the present invention is concerned with aspecific procedure for treatment of carbon with strong acids in a mannerwhich is not only effective to remove impurities, but also to alter thecomposition and surface characteristics in such a Way as to make thecarbon unique for demineralization usage.

Accordingly, it is an important object of the present invention toprovide a novel purified carbon, having a unique structure, as shown byits unexpectedly good action in an electrostatic demineralization cell.

A further object is to provide a novel process for treatmg carbon toremove impurities therefrom, and simultaneously alter the structurethereof.

A further object is to provide a novel electrode for use in anelectrostatic demineralization operation, and accordingly a novelelectrostatic demineralization process.

Other objects of this invention will appear in the following descriptionand appended claims. It is to be understood that the terminologyemployed is for the purpose of description and not of limitation.

1II IGURE 1 is a sectional view of a demineralization ce FIGURE 2 is agraphic illustration of sodium chloride removal from aqueous sodiumchloride;

FIGURE 3 is a graphic illustration of calcium chloride removal fromaqueous calcium chloride;

FIGURE 4 is a graphic illustration of calcium sulfate removal fromaqueous calcium sulfate; and

FIGURE 5 is a graphic illustration of the improvement provided by thepresent invention in calcium sulfate removal.

THE METHOD OF INVENTION Introduction Briefly, in accordance with thepresent invention, carbons containing silica and/ or graphitoidalsilicon residues, and other ash components, are treated with strongacids in a sequence of steps, followed by thorough washing. Itapparently is advantageous to thereafter dry the sotreated carbon inorder to complete the conversion of the structure to the novel form,before it is used in a demineralization cell.

There are two aspects of carbon treatment in accordance with the presentinvention, as follows:

(1) Silica and ash removal: The use of hydrofluoric acid, followed byboiling with hydrochloric acid; and

(2) Graphitoidal silicon and ash removal: Treating the carbon with amixture of hydrofluoric and nitric acids, followed by hydrochloric acidand water washes.

The two procedures may be used independently, or jointly, depending onwhether the carbon form contains both silica and graphitoidal silicon,or only one of these contaminants.

In view of the foregoing, the various aspects of the inventive processwill now be described in detail.

SILICA REMOVAL This aspect of the invention is shown by the followingexample.

Example I Pieces of type FC-20 carbon were obtained from the Pure CarbonCompany of St. Marys, Pa., of A thickness. This carbon has a surfacearea of 600 m. gm. and has a negligible resistance to the flow of waterthrough it in a demineralization cell. Other characteristics of thiscarbon are as follows:

Median pore diameter, microns 20 Scleroscope hardness 10-20 Specificresistance, ohm/cm. 0.2 Chemical properties:

Ash, percent 7 Sulfur, percent 0.01 Spectrographic on ash, selectedelements,

percent- SiO Major A1 10.0 Fe O 7.0 CaO 0.75 MgO 0.75 TiO 1.5 N320 3.0 K0 6.0

Pressure required to force nitrogen gas, 1 cmP/min. cm. through asection 1 cm. thick.

The carbon was treated as follows:

(1) The carbon pieces were immersed in a 70% aqueous solution ofhydrofluoric acid for a period of 16 hours.

(2) The carbon pieces were then transferred to a solution of 20.2%aqueous hydrochloric acid and boiled for a period of 1 hour.

The carbon pieces were then transferred to fresh aqueous hydrochloricacid of 20.2% concentration and the second step was repeated for a totalof 6 times.

(3) The carbon pieces were removed from the hydrochloric acid andimmersed in distilled water and boiled for /2 hour. This treatment wasrepeated for a total of 3 times. Substantially all of the hydrochloricacid was removed by so operating.

(4) The carbon pieces were dried in a hot air circulating oven at atemperature of 100 C. for a period of 16 hours.

(5) An optional step to assure removal of absorbed species, such aschlorides. The carbon pieces were then rinsed thoroughly with an aqueoussolution of about 4% hydrochloric acid.

(6) The carbon pieces were then immersed in distilled water and boiledfor /2 hour. This step Was repeated for a total of 2 times with boilingdistilled water.

(7) Finally, the carbon pieces were dried in a hot air circulating ovenat a temperature of 100 C. for a period of 16 hours.

After treatment this carbon contained .05 total ash, and wassubstantially free of SiO After this total treatment, the carbon pieceswere utilized as electrodes in a demineralization cell as describedbelow, operating with an aqueous calcium sulphate solution as the mediumto be deionized. Ion removal was excellent as indicated in FIGURE 5.

4 GRAPHITOIDAL SILICON REMOVAL This aspect of the invention is shown bythe following examples:

Example II-A Median pore diameter, microns 2.1 Pore void volume, cm. gm.carbon:

Macropores, -0035 microns 0.28 Micropores, lessthan 0.035 micron 0.24

Total 0.52

Porosity, volume percent 47 Flow resistance, mm. Hg/cm. 12 Physicalproperties:

Surface area m. gm 450 Average density, gm./cc. 0.90 Flexural strength,p.s.i 1,600 Scleroscope hardnes 35-45 Specific resistance, ohm/cm. 0.03Chemical properties:

Ash, percent 10.50 Sulfur, percent 0.01 Spectrographic on ash, selectedelements,

percent- SiO Major A1 0 0.8 F6203 CaO 0.5 MgO 0.2 TiO 0.02 Na O 2.0 K 0Pressure required to force nitrogen gas, 1 cmfi/min. cm. through a.section 1 cm. thick.

The carbon was treated as follows:

(1) The pieces of carbon were immersed in a mixture of 20-25% nitricacid and 7075% hydrofluoric acid for 16 hours.

(2) The carbon pieces were subjected to 6 successive 1 hour boilingtreatments with 20.2% aqueous hydrochloric acid.

(3) The carbon pieces were immersed in distilled water and boiled for /2hour, for a total of three separate fresh water treatments.

(4) The pieces were dried at 100 C. for 16 hours.

(5) The pieces were rinsed thoroughly with about 4% aqueous hydrochloricacid.

(6) The pieces were then immersed in distilled water and boiled for /2hour, for a total of two treatments.

(7) Finally, the carbon pieces were dried in a hot air circulating ovenat a temperature of 100 C. for a period of 16 hours.

After treatment this carbon contained 0.24% ash, which included 0.04%SiO and less than 0.01% Si, based on total material. The remainder ofthe ash comprised trace amounts of A1 0 Fe O CaO, MgO, TiO and Na O.

Example I-I-B This example illustrates that hydrochloric acid treatmentis ineffective to remove graphitoidal silicon. Pieces of Purebon FC-13carbon, as in Example II-A, were utilized. Before treatment this carboncontained 10.5% ash, including 4.97% SiO and 1.50% graphitoidal silicon,all based on total material.

The pieces were first immersed in hydrofluoric acid of 70% concentrationfor 16 hours.

The pieces were then removed and placed in 20.2%

aqueous hydrochloric acid and boiled for V2 hour. This treatment wasrepeated for a total of 3 times.

The pieces were then immersed in distilled water and boiled for /2 hour,for a total of three treatments.

The pieces were then dried for 16 hours at 100 C.

Analysis after treatment showed about 0.1% SiO and graphitoidal siliconof 1.50%. There was a total of 1.64% ash after this attemptedpurification.

It was clearly evident that this treatment did not remove graphitoidalsilicon.

DISTINCTIONS FROM THE PRIOR ART To test the effectiveness of gaseouschlorine on the removal of impurities, a sample of FC-l3 carbon waschlorinated at 1000 C. for 16 hours. The total silicon content,including that of Si and graphitoidal Si, was reduced only slightly,from 5% to about 3%,,leaving considerable ash.

-By the process of the present invention, on the other hand, the samecarbon had the entire silicon content, and substantially all of the ash,removed.

THE DEMINERALIZATION ASPECT Example III A demineralization cell wasfabricated as represented in schematic form in FIGURE 1. The raw carbonused in Example I, namely FC-20 a high surface area material from PureCarbon Company, St. Marys, Pa., was used for the electrodes. The carbonhad a surface area of 600 mi /gm. with a negligible resistance to flowof water through it. The cell comprised a plurality of electrodes fls"thick, each cemented to a graphite rod current collector. Glass fiberseparators were used between electrodes. The electrodes were stacked andencased in an epoxy resin container, sealed on the outside with siliconerubber. "Methyl methacrylate (Lucite, trademark) top and bottom platescompleted the unit.

The cell was utilized for the removal of ionic materials from hard wellwater. Opposite electrical charges were imposed on adjacent electrodes.It was found that the electrodes quickly plugged up to such an extentthat no Water could be pumped through the cell.

In view of this failure, several runs were made to demineralizesynthetic hard Water solutions containing single components of theoriginal well water in order to determine which ionic component, orcombination of ionic components, was responsible for the electrodeplugging.

Sodium chloride: -An identical cell was constructed, using the raw FC-20carbon for electrode material. Thirty (30) /8 electrodes were used toprovide a column volume of 40 cc.

This cell was used for the demineralization of an aqueous solutioncontaining 300 ppm. of sodium chloride. The cycle time for celloperation was 1 hour; comprising, charging and discharging times of 20minutes each, and pumping time of 10 minutes.

Results are shown in FIGURE 2 of the drawings. The numerals 44, 106 and120 refer to cycle numbers. The concentration of sodium chloride in theeffiuent demineralized water is approximately 350 p.p.m., which is wellbelow the average concentration of about 500 p.p.m. of sodium chloridein fresh water.

It will be noted that the eifectiveness of the cell continues to improveeven after 120 cycles.

Calcium chloride: An identical cell was constructed using raw FC-ZOcarbon for electrodes. Twenty (20) A5" electrodes were used to provide acolumn volume of 20 cc.

This cell was used for the demineralization of an aqueous solutioncontaining 250 p.p.m. of calcium chloride with the results shown inFIGURE 3. In this case the graph shows the composition of the waterremoved after typical charge and discharge cycles. Again excellent ionremoval was obtained. The efiluent demineralized water had aconcentration of only 25 p.p.m. of calcium chloride which can befavorably compared with a concentration of about p.p.m. of calciumchloride in a typical municipal water supply.

Calcium sulfate: Drastic ion removal decrease. Results observed using ademineralization cell similar to that depicted in FIGURE 1, operating onan aqueous solution of calcium sulfate are shown in FIGURE 4. Ionremoval was observed as decreasing rapidly with each cycle; in fact, ionremoval was so poor as early as the third cycle that the operation wasterminated. These results provided a strong indication that a majorsource of the difiiculty encountered in the attempted demineralizationof natural well Water was related to the calcium sulfate component.

INVENTION OF TREATED CARBON ELECTRODES Many expedients were investigatedin an attempt to improve the removal of calcium sulfate from aqueoussolution, including the reversal of positive and negative electrodes,adjustment of the discharge potential of the cell, and the utilizationof various types of carbon as electrode materials. However, none ofthese were successful. It has been found unexpectedly that theacid-treated carbon of this invention provides satisfactory removal ofcalcium sulfate.

Example IV Novel carbon usage: A cell was constructed similar to thecell diagrammed in FIGURE 1, with the exception that the electrodes werefabricated of A" thick pieces of type FC-20 carbon which had beenpretreated with hydrofluoric and hydrochloric acids according to theprocess of Example I. The results of this cell operating on an aqueouscalcium sulfate solution are shown in graphic form in FIGURE 5. Thenumerals 32, 43 and 62 refer to the number of cycles of operation. It isquite apparent that the cell is functioning as effectively in removingcalcium sulfate after 62 cycles of operation as it was initially. Thusthe practicability and utility of the method of the present inventionare amply demonstrated.

Example V This example illustrates that FC-13 carbon is also improvedfor demineralization usage by treatment in accordance with the presentinvention.

Two sets of electrodes were prepared from Purebon FC-l3 carbon, 1" x 3"x This carbon is described in Example II-A above. One set was purifiedas follows:

(1) The carbon pieces were immersed in a mixture of 20-25% HNO and70-75% HF for approximately 16 hours, followed by:

(2) Six successive one hour boilings with 20.2% aqueous HCl.

(3) The carbon pieces were then immersed in distilled water and boiledfor /2 hour for a total of three separate fresh water treatments.

(4) The pieces were then dried at C. for 16 hours.

(5) Pieces immersed in 4% aqueous HCl.

(6) Pieces immersed in distilled water and boiled for 2 hours for atotal of 2 treatments.

(7) Pieces dried at 100 C. for 16 hours.

This process is slightly different from that described in Example II-A.

Identical pieces of purified and unpurified electrodes were placed in 2Normal aqueous KCl for 196 hours soaking time. The electrodes were thencycled between 0-1 volt for approximately 30-40 cycles. Capacity studieswere then determined. The result was that the purified electrodes hadtwice the capacity of the unpurified electrodes.

In summary, tests on unpurified and purified FC-l3 carbon electrodes inaqueous KCl showed that the purified FC-13 resulted in twofoldimprovement in demineralization over the identical unpurified FC-13.

What is claimed is:

1. A process for purifying carbon containing silicon comprising thefollowing steps, in order:

7 8 immersing said carbon in a mixture of concentrated OTHER REFERENCESaqHeCPuS i mtnc acld Strasheim: Chemical Abstracts, vol. 43, 1949, col.lmmerslng said carbon in boiling aqueous hydrochloric 5336 5337 9Parisot et 211.: Chemical Abstracts, v01. 46, 1952, col. I'lIlSlDg saidcarbon with water, and 5 5973 drymg Sald carbon- Kirk-OthmerEncyclopedia of Chemical Technology,

2. A process as in claim 1 wherein the aqueous hydro- 1 4 d 4 18 fluoricacid is of about 70% concentration. v0 2nd 6 196 9 3. A process as inclaim 1 wherein the aqueous hydro- EDWARD MEROS, Primary Examinerchloric acid is of about 20% concentration. 10

4. A process as in claim 1 wherein the aqueous hydro- I U S c1, X Rchloric acid is heated at its boiling point. 136121 204149 ReferencesCited FOREIGN PATENTS 15 23,233 1899 Great Britain. 17,731 1913 GreatBritain.

